(1) Field of the Invention
The present invention relates to a process for the preparation of hydroperoxides of aromatic compounds having a secondary alkyl group. More particularly, the present invention relates to an improvement of this process in which inactivation of a copper compound catalyst supplied to the reaction system owing to insolubilization thereof can be effectively prevented even under basic conditions.
(2) Description of the Prior Art
A process in which an aromatic compound having a secondary alkyl group, such as cumene, cymene, dimethylcumene or sec-butylbenzene, is oxidized with molecular oxygen to form a corresponding hydroperoxide has been previously known. Various processes for promoting this oxidation reaction by using heavy metal catalysts have been proposed, and certain effects have been attained by these processes. In most of these processes, a heavy metal catalyst is dissolved or suspended in the above-mentioned aromatic compound and oxidation is then carried out. However, they are still defective in various points. For example, the oxidation reaction speed is not sufficiently high and the selectivity to the intended hydroperoxide is low. Furthermore, a reaction vessel or the like is corroded by an organic acid formed as the by-product, and the heavy metal catalyst is inactivated because of insolubilization thereof. As means for preventing corrosion by the organic acid formed as the by-product, there may be considered a process in which oxidation is carried out in the co-presence of a basic aqueous solution. However, in such a basic aqueous solution, the heavy metal catalyst undergoes a chemical change and it is insolubilized, and there is a risk of inactivation of the catalyst. Accordingly, such a process has hardly been proposed. A process using an organic chelate compound of a heavy metal has been proposed in Japanese Patent Application Laid-Open Specification No. 142526/75 or No. 142527/75. However, preparation of this catalyst is not easy and this catalyst is very expensive. In the process disclosed in Japanese Patent Application Laid-Open Specification No. 142527/75, a precipitate is formed and the catalyst is inactivated unless the pH value of the aqueous phase is maintained below 7.5, as described in this Laid-Open Specification. Thus, the operation under basic conditions is remarkably limited. If an organic ligand having a high coordination power is used, the heavy metal ion can be kept water-soluble even at a high pH value. However, if a metal chelate compound containing such a strong organic ligand is used for the oxidation reaction, a problem arises in connection with the activity of the catalyst and there is a risk of reduction of the quality of the intended product by this ligand.